Ive just has a look at the wiring diagrams and from the "Monolever" stalk to the "Main Electromagmetic Horn" :lol: is a simple orange wire which runs from connection A4 on the switch connector through fuse F38 then changes colour to violet to the horn and then to chassis earth.... So, the horn does work, so its not going to be anything between the switch and the horn.
The rotary "Monolever" has three electrical feeds. Confusingly labled
F21 (20amp)- Horn and lights switch
F25 (5amp)- Horn and lights switch
F15 (5amp)- Horn and lights switch
Looking at the functions of the wires, it looks like F21 is the only one actually feed the horn but I can't be certain how the switch is configured inside and it will take ages to work around the diagrams
Check the three fuses listed above (and F38) are sound, with clean legs and fitted correctly to get a good contact. It takes a fair ammount of power to drive the horn and if some of the current is being limitted by a poor connection....
If that hasn't curred it, pull the fuses that go from the switch to the lights. F32, F33, F34, F35 (added F42 and F43 sidelights). That way, theres not going to be any extra electrical load on the horn curcuit from the lights being lit. So, if the horn still plays up, the fault its very likely to be within the switch or the connector onto the switch